Geologic Provinces of the United States: Ouachita-Ozark Interior Highlands

Ouachita-Ozark Highlands

The ancient, eroded mountains of the Ouachita-Ozark Highlands stand surrounded by the nearly flat-lying sedimentary rocks and deposits of the Interior and Atlantic Plains provinces. Unlike the relatively young rocks that characterize neighboring provinces, the rocky outcrops that make up the core of the Ouachita-Ozark Highlands are Paleozoic age carbonate and other sedimentary rocks that were originally deposited on the sea floor. In the Ouachita Mountains these ancient marine rocks are now contorted by folds and faults. These rocks closely match deformed strata found today in the Marathon Mountains of Texas and the southern Appalachians—strong evidence that the Ouachita-Ozark Highlands were once part of a mighty folded, uplifted mountain range that stretched from the Appalachians Highlands to the northeast through Texas to the southwest.

Big Spring, Ozark National Scenic Riverways. Big Spring is the largest spring in Missouri. This spring is one of a number of large springs emanating from Paleozoic limestone bedrock the Ozark Plateaus Province. Photo by Randy Orndorff, USGS.

Quiet before the storm

For many millions of years, from the end of the Precambrian to the Early Mississippian, the Ouachita-Ozark Highlands region lay submerged beneath the sea. Along this tectonically quiet, passive margin, formed by the earlier breakup of a supercontinent, sediment eroded from the land and was gradually carried to the sea floor. Thousands of feet of carbonate, sand, and finer grained material loaded onto the submerged continental margin.

Collision course

By 340 million years ago, early in the Mississippian, the passive coastal margin changed to an active convergent boundary. The southeast coast of North America was on a collision course with a small plate once connected to Africa and South America. For tens of millions of years thrust faults and folds piled up marine sediment and rock, building the Marathon-Ouachita-Appalachian mountain chain, one of the final events in the formation of the supercontinent, Pangaea.

Eventually continental collision ceased and the exposed, uplifted rock began to feel the effects of weathering and erosion.

Ripping the range

By 200 million years ago (Jurassic), Pangea was breaking up in a big way. South America tore away from North America and headed southward. The ocean flooded into the opening between the two continents, forming the Gulf of Mexico. A record of this rifting event remains as an indelible mark on the landscape called Mississippi Embayment. It is this embayment that ripped the dramatic gap between the southern Appalachians and the Ouachita-Ozark Highlands.

By the end of the Mesozoic Era, most of the once-mighty Marathon-Ouachita-Appalachian mountain chain had eroded. Today, only a small part of the original range, rejuvenated in the Cenozoic Era, remains.